In general, the two categories of multiplexers are synchronous and asynchronous. Both types combine two or more serial data streams called groups into a single higher rate serial data stream called a supergroup. In a synchronous multiplexer, each group is assigned a fixed fraction of the bits in the supergroup. Thus, if a supergroup output rate is 10 kilobits per second and a group input rate is 1 kilobit per second, exactly one-tenth of the supergroup bits would come from that group. In order to provide uninterrupted operation with a synchronous multiplexer, the data rate of each group must be maintained in an exact ratio with the data rate of the supergroup. This is usually achieved by phase-locking the data rate frequency of each group with a submultiple of the supergroup data rate frequency.
In some applications, it is impractical to phase-lock the group clocks to the supergroup clock. In these applications, an asynchronous multiplexer is generally used. Because a group rate is not necessarily a fixed fraction of the supergroup rate in an asynchronous multiplexer, it is not known exactly how many bits that group will contribute to a frame of the supergroup. Conventionally, this uncertainty may be compensated for through the use of so-called stuff bits. More specifically, the group rate is generally specified to be within a certain range of several hundred parts-per-million of a center frequency. Accordingly, because the lowest possible frequency of the group clock is known, the fewest number of bits contributed by a group to a supergroup frame can be determined. In other words, it is known how many bits of a group will always be transmitted as part of a supergroup frame and these bits are assigned data bit positions in the frame. If the group clock frequency is higher than the lowest frequency of its range, additional bits must be transmitted for that particular group. These bits are called stuff bits and data bit positions in the supergroup frame are assigned for them. Enough data bit positions must be assigned to accommodate for the group clock frequency being at the upper end of its specified range. It follows that depending on the frequency of the group clock rate, the stuff bit positions in the frame may or may not be used for group data. Because the demultiplexer must provide only the stuffed data bits from these bit positions to a particular device, the multiplexer must also send a code in the supergroup output indicating whether the frame data bit positions assigned to the stuff bits have been used or not.
Certain multiplexer-demultiplexer applications such as, for example, tropospheric-scatter communication links are subject to periodic fades, that is, intervals in which the bit error rate is significantly poorer than the long term average. During the fade, data may be lost or garbled but what is more important is that after the fade, the bit count integrity must be achieved. Bit count integrity on the group means that the number of bits received in the group at the demultiplexer is equal to the number of bits transmitted in that group at the multiplexer over a given interval of time. Loss of the bit count integrity of group data is very undesirable. Many user types of equipment generally used with demultiplexer must go through a lengthy resynchronization procedure when the bit count integrity is lost. In some cases, this resynchronization must be initiated manually. The net effect of bit count integrity losses is to substantially reduce the percentage of time a communication link is available.
With a synchronous demultiplexer, the bit count integrity of the groups is maintained as long as the bit count integrity is maintained on the supergroup. This is because each group rate is a fixed ratio of the supergroup rate. These ratios are designed into the demultiplexer and do not depend on the transmission of information from the multiplexer. With an asynchronous demultiplexer, however, the bit count integrity of a group is dependent both on the bit count integrity of the supergroup, and on the correct interpretation of stuff codes which are subject to transmission errors. Accordingly, previous asynchronous demultiplexers were not suited for an environment where severe fades result in the incorrect interpretation of the stuff codes.